Today, different technologically important wavelength regimes are detected by separate photoactive semiconductors with appropriate bandgaps. For example, GaN, Si and InGaAs are typically exploited for sensing in ultraviolet, visible and near-infrared regimes, respectively. Detection of mid-infrared photons generally relies on small-bandgap semiconductor compounds such as HgCdTe, PbS or PbSe, while thermal sensing techniques are utilized for detection in far-infrared regime. In contrast, graphene is a promising optoelectronic material for ultra-broadband photodetectors due to its gapless band structure. The difficulty with utilizing graphene in standard photodetector structures is that the lifetime of photo-generated carriers is very short; it is thus necessary to separate the electrons and holes on a sub-picosecond time scale in order to efficiently generate a photocurrent and avoid simple heating of the graphene layer. So far, nearly all graphene-based photodetectors focus on exploiting graphene-metal junctions or graphene p-n junctions for extracting photocurrent. Unfortunately, these sensing schemes suffer from a small area of the effective junction region contributing to the photocurrent, along with weak optical absorption; the responsivity is thus limited to a few mA/W. Integrating graphene with plasmonic nanostructures or microcavities can enhance the light-graphene interaction and improve responsivity to tens of mA/W; however, the enhancement can only be achieved at the designed resonant frequencies, restricting their applications for broadband photodetection. In this regard, the idea of silicon waveguide-integrated graphene photodetectors was proposed recently, showing broadband photodetection with the enhanced responsivity to tens of mA/W. Photoresponsivity above 0.1 A/W can also be achieved in transition metal dichalcogenide/graphene stacks by exploiting the strong light-matter interaction. Band structure engineering in graphene has also recently been explored for photoresponsivity enhancement, but efficient photodetection can only be achieved below about 150 K due to the short electron life time in midgap states at elevated temperatures.
An alternative approach is to exploit photoconductive gain in graphene. Although graphene is conventionally regarded as a poor photoconductor owing to its ultrafast hot carrier recombination, recent studies demonstrated that hybridized graphene/quantum-dot photodetectors can achieve high photoconductive gain. This sensitive detection scheme is attributed to a strong photo-gating effect induced by trapped photocarriers in the quantum dots. Despite the excellent device responsivity, light absorption relies on the quantum dots instead of graphene, thus restricting the spectral range of photodetection.
In this disclosure, a graphene-based ultra-broadband photodetector is presented. In contrast to conventional phototransistors as well as lateral graphene devices, hot electrons and holes are separated in the proposed structure by selective quantum tunnelling into opposite graphene layers, thereby minimizing hot carrier recombination. The trapped charges on the top graphene layer can result in a strong photo-gating effect on the bottom graphene channel layer, yielding an unprecedented photo-responsivity over an ultra-broad spectral range. Furthermore, by engineering the proper tunnel barrier, prototype devices achieving ultra-broadband photodetection and a room temperature mid-infrared responsivity comparable with the state-of-the-art infrared photodetectors operating at low temperature are demonstrated.
This section provides background information related to the present disclosure which is not necessarily prior art.